Iron-, nickel-, chromium base alloy

ABSTRACT

An iron, nickel-, chromium base alloy having an austenitic structure, good high temperature features, including a very high resistance to oxidization in an oxidizing atmosphere and to carburization in a carborizing atmosphere at high temperatures, and a high creep fracture resistance. The alloy has the following composition in weight percent: 0.01-0.08 carbon, 1.2-2.0 silicon, from traces up to 2 manganese, 22-29 chromium, 32-38 nickel, 0.01-0.15 rare earth metals, 0.08-0.25 nitrogen, with the balance essentially of only iron and unavoidable impurities and normally occurring accessory elements in normal amounts. The rare earth metals in combination with the silicon content serve to improve the growth of a protecting silicon dioxide-layer on the metal surface, when the metal surface is subjected to high temperatures in oxidizing atmospheres. This counteracts the transportation of metal irons, in particular chromium, out of the alloy so that scaling is minimized.

TECHNICAL FIELD

The present invention relates to an iron-, nickel-, chromium base alloyhaving an austenitic structure and good high temperature features,including a very high resistance against oxidization in oxidizingatmosphere and against carburizing in carburizing atmosphere at hightemperatures, as well as a high creep fracture resistance.

BACKGROUND OF THE INVENTION

High alloyed, stainless, austenitic steels or nickel base alloyscontaining up to 60% nickel conventionally have been used for objectswhich during a long period of time are subjected to high temperatures incombination with mechanical loading in oxidizing environments. Thesealloys usually have a high oxidization resistance and often also a veryhigh creep fracture resistance, but because of the increasingly highdemands which are raised upon materials for the present field of usethere has arisen a need of materials having still better oxidizationresistance in oxidizing environments in combination with very good creepfracture resistance, a combination of features which has notsatisfactorily been achieved with presently known alloys.

Another problem with known alloys of the above mentioned kind is thatthey have a comparatively great tendency to take up carbon and nitrogenwhen exposed in carburizing atmosphere or in environments which involvea risk for the taking up of nitrogen at high temperatures. Thisparticularily concerns austenitic steels but to an essential degree alsonickel base alloys. Also attacks from gaseous halides and metal oxidesin certain environments may involve problems.

The above mentioned problems will be particularily accentuated in thosecases when the material is subjected alternatingly to carburizing and tooxidizing media at high temperatures, or, which sometimes even mayoccur, in environments which at the same time may act oxidizing as wellas carburizing. Those situations when the material in hot condition isexposed to ambient air after having been subjected to carburizing in anfurnace at a high temperature are examples of alternatingly carburizingand oxidizing exposures. Similar conditions may occur in furnaces whereit from some reason is difficult to maintain a balanced atmosphere.Further may be mentioned furnace linings which are subjected to cokedepositions. It is conventional to remove such depositions by burningthem off, wherein air is supplied for the combustion, which is a furtherexample of exposure to alternatingly carburizing and oxidizing media.Finally, treatment of poorly degreased goods in oxidizing atmosphere athigh temperatures is an example of a situation where carburizing andoxidizing may occur at the same time.

DESCRIPTION OF THE INVENTION

The invention aims at providing an alloy having a composition whichbrings about an improved resistance at high temperatures againstcarburizing as well as against oxidizing, and which also gives a goodcreep fracture resistance. The material according to the invention alsohas a good resistance to the taking up of nitrogen and also has goodresistance to attack from gaseous halides and metal oxides. It canadvantageously be used in the form of sheets, plates, bars, rods, wiresand tubes in various kinds of furnaces, as for example carburizingfurnaces, sintering-, annealing-, and tempering stoves, where also nondegreased goods are heat-treated, and it can also be used foraccessories for furnaces and stoves, for example charging-baskets,-grates and -buckets. Further it can be used in burners, combustionchambers, radiant-tubes, reaction rooms in the petrochemical industryand in fluidized beds, exhaust gas filters for motor cars, etc.

The following table shows the broad range for the elements which areincluded in the alloy according to the invention, and also thepreferred, and the suitably chosen ranges. The contents are expressed inweight-%. The balance is iron, unavoidable impurities in normal amountsand normally existing accessory elements. For example there is anegligible amount of aluminum and calcium in the steel as a rest duefrom the finishing metallurgical operation prior to casting. Thecontents of phosphorus and sulphur are very small, max 0.04%, and max0.008%, respectively.

                  TABLE 1                                                         ______________________________________                                                Broad    Preferably Preferred                                                 ranges   chosen ranges                                                                            composition                                       ______________________________________                                        C         0.01-0.08  0.02-0.08  0.035-0.065                                   Si        1.2-2.0    1.3-1.8    1.3-1.8                                       Mn        from traces to max 2                                                                            1.3-1.8                                           Cr        22-29      23-27      24-26                                         Ni        32-38      33-37      34-36                                         Rare earth                                                                              0.01-0.15  0.02-0.12  0.03-0.10                                     metals                                                                        N         0.08-0.25  0.1-0.2    0.12-0.18                                     ______________________________________                                    

The carbon content has importance for the features of the steel, as faras the strength is concerned, and shall therefore exist in an amount ofat least 0.01%, preferably at least in an amount of 0.02%, and suitablynot less than 0.035%. If the alloy shall be used for the production ofplates, sheets, rods, wires, and/or tubes, the carbon content, however,should not exceed 0.08%, suitably not exceed 0.065%.

Silicon is required in an amount of at least 1.2% in order that acombination effect between silicon and the rare earth metals shall beachieved with reference to the oxidization resistance. This will beexplained more in detail in connection with the description of thecerium content. Silicon also is favourable for the carburizingresistance. From these reasons, the silicon content should be at least1.3%. The upper silicon limit, 2.0%, preferably max 1.8%, is due tocircumstances which has to do with technical circumstances relating tothe manufactoring and also to the fact that higher silicon contents maycause difficultes in connection with welding.

Manganese generally improves the strength but impaires the oxidizationresistance. The content of manganese therefore should not exceed 2% andshould suitably be 1.3-1.8%.

Phosphorous and sulphur in amounts exceeding the above mentioned maximumlimits have an unfavourable influence upon the hot workability.

The chromium content is high and lies within the range 22-29%,preferably 23-27%. Herethrough there is achieved, in combination with ahigh nickel content, a high silicon content, and a significant contentof rare earth metals, a good resistance against high temperaturedamages, in the first place against carburizing and oxidization at hightemperatures.

Nickel is favourable for the oxidaization resistance and also for thecarburization resistance and shall exist in an amount between 32 and38%, preferably in an amount between 33 and 37%. A preferred compositionis 34-36%.

Rare earth metal in the form of the lanthanum group of metals in anamount, expressed in the amount of cerium which normally stands forabout 50% of the mischmetal, of 0.01-0.15%, preferably at least 0.02%,and suitably at least 0.03% cerium, improves the formation of a thin,elastic and adhering oxide film, when the alloy according to theinvention is exposed to an oxidizing environment at high temperatures.However, there is not obtained any further improvement of theoxidization resistance in proportion to the addition of rare earthmetals, if the content of rare earth metals, in the first place cerium,exceeds 0.12%. The preferred range for the amount of rare earth metaltherefore lies between 0.03 and 0.10%. Possibly the rare earth metalscompletely or partly may be replaced by earth alkali metals.

Cerium and other lanthanides (rare earth metals) are suitably suppliedas mischmetal to the finished molten alloy together with silicon-calciumor possibly lime as a final operation. Through the addition of siliconcalcium and/or by covering the melt with a layer of lime it is possibleto prevent major losses of cerium and other rare earth metals, so thatthe rare earth metals, as expressed in amount of cerium, will exist in asufficient amount in the finished product in order to bring about thedesired effect. Through the influence of cerium and other rare earthmetals in the mentioned range of composition there will in combinationwith silicon in the above mentioned range of composition be achieved afavourable impact upon the growth of a SiO₂ -layer on the metal surface,when the metal surface is subjected to high temperatures in an oxidizingenvironment. This SiO₂ -layer will form a barrier against thetransportation of metal ions, in the first place chromium, out of thealloy, so that scaling is minimized.

Nitrogen has a favourable influence upon the creep fracture strength ofthe alloy and shall therefore exist in an amount of at least 0.08%,preferably at least 0.1%, and suitably at least 0.12%. Nitrogen,however, at the same time impaires the hot workability of the alloy andshall therefore not exist more than in a maximum amount of 0.25%,preferably max 0.2%, and suitably max 0.18%. Moreover, there may existtraces of other elements, however, not more than as unavoidable amountsof impurities or as accessory elements from the melt metallurgicaltreatment of the alloy. Thus the steel may contain a certain amount ofcalcium and aluminum as a residual product from the finishing of thesteel. Boron is an example of an element that shall be avoided, sincethat element even in very small amounts may impaire the oxidizationresistance of the alloy by locating itself in the grain boundaries,where the existence of boron may prevent oxygen from penetrating and bedeposited in the grain boundaries in a form of oxides.

BRIEF DESCRIPTION OF DRAWINGS

In the following description of the results, reference will be made tothe attached drawings, in which

FIG. 1 is a graph in which the results after intermittent oxidizationannealing of a number of commercial alloys are compared with the resultsfrom a first example of an alloy according to the invention, and

FIG. 2 is a graph which illustrates the oxidization resistance of analloy according to a second example of the invention by showing theincrease of weight in a thermo-balance as a function of the annealingtemperature up to 1300° C.

OXIDIZATION EXPERIMENTS

In Table 2, alloys 1-7 are examples of the invention. Alloys A, B and Care commercial reference alloys. Alloy 1 was manufactured as a 500 kgtest charge. Alloys 2-6 were manufactured as 13 kg laboratory charges.Alloy 7 was manufactured as a 10 ton full scale charge. As far as alloys1-6 are concerned, the molten alloy was analysed prior to casting aswell as the composition of the finished product. The impurity contentsin all the examples were low. The balance therefore consistedessentially only of iron. The compositions of alloys A, B and C wereobtained from the specifications for these materials.

                                      TABLE 2                                     __________________________________________________________________________    Alloy                                                                             Charge/                                                                   No  product                                                                            C  Si  Mn Cr  Ni  Ce N      Remarks                                  __________________________________________________________________________    1   052875                                                                             0.058                                                                            1.27                                                                              1.58                                                                             25.1                                                                              34.7                                                                              0.05                                                                             0.033                                               plate                                                                              0.054                                                                            1.19                                                                              1.59                                                                             "   "   "  0.032                                           2   B322 0.045                                                                            1.75                                                                              1.68                                                                             24.7                                                                              34.7                                                                              0.065                                                                            0.126                                               bar  "  "   1.67                                                                             25.0                                                                              34.9                                                                              0.03                                                                             0.121                                           3   B325 0.049                                                                            1.56                                                                              1.55                                                                             25.0                                                                              34.8                                                                              0.086                                                                            0.55                                                bar  "  1.54                                                                              1.53                                                                             "   "   0.034                                                                            0.56                                            4   B323 0.047                                                                            1.55                                                                              1.43                                                                             24.7                                                                              34.8                                                                              0.053                                                                            0.146                                               bar  "  1.52                                                                              1.42                                                                             "   34.9                                                                              0.018                                                                            0.147                                           5   B321 0.047                                                                            1.78                                                                              1.67                                                                             24.7                                                                              34.7                                                                              0.059                                                                            0.077                                               bar  0.046                                                                            1.75                                                                              1.66                                                                             25.0                                                                              34.9                                                                              0.023                                                                            0.078                                           6   B320 0.040                                                                            1.87                                                                              1.80                                                                             24.9                                                                              35.3                                                                              0.114                                                                            not analysed                                        bar  "  1.83                                                                              1.78                                                                             "   "   0.034                                                                            0.022                                           7   2281-71                                                                            0.048                                                                            1.52                                                                              1.74                                                                              25.75                                                                            34.6                                                                              0.045                                                                            0.130                                               plate                                                                     A        max                                                                              max max                                                                              24-26                                                                             19-22                                                           0.08                                                                             1.5 2.0                                                           B        0.04                                                                             0.35                                                                              0.75                                                                             21  31            0.3 Cu                                   C        max                                                                              1.5-                                                                              0.5                                                                              21  11  0.05                                                                             0.15                                                     0.10                                                                             2.3                                                               __________________________________________________________________________

The oxidization resistance of alloy No 1 was examined throughoxidization annealing. Test coupons 25×15×2 mm were taken out from theplate. The coupons were planed and ground. The test coupons wereoxidization annealed during a total annealing time=45 h and with fivealternations down to room temperatures. The test coupons were annealedat varying temperatures between 1050° and 1200° C. The coupons wereweighed by means of a standard balance prior and after the annealingexperiments. The results are shown in FIG. 1 which also includes theresults from corresponding testing of the commercial alloys A, B and C.From these results it can be stated that the scaling temperature may be1200° C.

Thereafter also the full scale produced alloy No. 7 was oxidizationtested in a thermo-balance. The increase of weight was measured as afunction of the annealing temperature as in the proceeding experimentbut all the way up to 1300° C. The coupons were weighed with a standardbalance prior and after the annealing experiments as a complement to thethermo-balance measurements.

The thermo-balance value and the differences between the coupon priorand after the experiment for each individual sample is shown in Table 3.

The increase of weight in the thermo-balance as a function of theannealing temperature is shown in the graph in FIG. 2. The limits 1.0and 2.0 gr/(m² h) has been indicated by a dashed line in FIG. 2 from thereason that the scaling temperature is defined by the size of theincrease of weight in the following way: "The scaling must not exceed 1g/(m² h) with the additional condition that 50° C. higher temperaturemust not give more than at the most 2 g/(m² h).

The result from the testing of alloy No. 7 shows that the alloy of theinvention resists also a scaling temperature above 1200° C.

                  TABLE 3                                                         ______________________________________                                        Table over each individual sample of alloy No. 7, 17.7                        mm plate, charge 2282-71. Intermittent annealing; five                        alternations during 45 h.                                                     Test                T-balance Loss of                                                                              Total take                               temperature                                                                            Experiment values    weight up of O.sub.2                            °C.                                                                             No.        g/m.sup.2 g/m.sup.2                                                                            g/m.sup.2                                ______________________________________                                        1100     B451        7.43      6.64  14.08                                    1150     B452        7.80     21.24  29.04                                    1200     B453       11.87     23.08  34.95                                    1200     B454       18.65     19.56  38.21                                    1250     B455       54.19     32.09  86.28                                    1250     B458       61.94     27.15  89.09                                    1300     B456       35.95     47.90  83.85                                    1300     B457       56.57     42.22  98.79                                    ______________________________________                                    

CREEP FRACTURE STRENGTH EXPERIMENTS

In these experiments the same alloys were used as in the oxidizationexperiments, Table 2.

The creep fracture strength of a 20 mm plate made of alloy No. 1 from a500 kg test charge was examined at the temperatures 600°, 750° and 900°C. Table 4 shows obtained R_(km) -values and (within brackets) referencedata including min/max-data from three full scale charges of thecommercial steel grade C, Table 2. The examined test material with thelow nitrogen content as expected has lower values than alloy C, which isknown to have an extremely high creep fracture strength.

                  TABLE 4                                                         ______________________________________                                        Temp  Creep fracture limit, R.sub.km, N/mm.sup.2                              °C.                                                                          10.sup.2 h 10.sup.3 h                                                                             10.sup.4 h                                                                             10.sup.5 h*                                ______________________________________                                        600   250        175      105      62                                               (300-315)  (235-240)                                                                              (145-155)                                                                              (≃88- ≃100)                                       8                                          750    78        45        24      13                                               (105-125)  (67-73)  (38-42)  (≃21- ≃24)                                        2                                          900    28        16        10       5                                               (36-40)    (23)     (14-16)  (≃8- ≃12)      ______________________________________                                         *The values for 10.sup.5 h have been derived through manual (graphical)       extrapolation about one 10power of time.                                 

The five 13 kg laboratory charges, alloys 2-6, were manufactured inorder to examine the impact of the nitrogen content upon the creepfracture strength of the alloy according to the invention. The ingotsfrom these small laboratory charges were forged to size φ 20 mm. Thenitrogen contents varied from min. 0.022% to max. 0.147%. The measuredcreep fracture limit values at 900° C. are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        N         Ce     Creep fracture limit, R.sub.km, N/mm.sup.2                   Charge                                                                              %       %      R.sub.km /100 h                                                                       R.sub.km /1000 h                                                                      R.sub.km /10 000 h*                      ______________________________________                                        B 322 0.121   0.030  33      20      (12)                                     B 325 0.056   0.034  31      19      (11)                                     B 323 0.147   0.018  34      18      (10)                                     B 321 0.078   0.023  33      17       (9)                                     B 320 0.022   0.034  28      16       (9)                                     ______________________________________                                         *The values for 10.sup.4 h have been derived through manual (graphical)       extrapolation about one 10power of time.                                 

In the continued experiments concerning the influence of the content ofnitrogen, the best result was achieved with alloy No. 2 containing 0.12%N. The improvement as far as the value of the creep fracture limit at900° C. is concerned was about 20%. The experiments also show that alsothe content of cerium appears to have an impact upon the creep fracturestrength. The comparatively low values for alloy No. 4 --in spite of anitrogen content of about 0.15%--therefore may depend on the fact thataccording to the control analyse the content of cerium was only 0.018%.This also indicates the importance of protecting the lanthanides duringthe manufacturing so that these elementes are not lost in connectionwith the finishing of the melt and the subsequent casting. Also the rodmaterial of alloy No. 5, which contained about 0.08% nitrogen and 0.023%cerium, seems to get a larger reduction of the creep fracture valueswhen the testing period is prolonged, probably depending on the moderatecontent of cerium, which indicates that the content of cerium should beat least 0.03% in order to bring about an effect not only upon theoxidization resistance but also upon the creep fracture strength. Theinvestigation moreover shows that the creep fracture strength issignificantly increased with increased nitrogen content.

CARBURIZATION EXPERIMENTS

These experiments concern studies if six different alloys in a reducing,carburizing atmosphere. The depths of carburization were measured andfrom these measurements the carburization rates were evaluated. Thechemical compositions in weight-% are shown in Table 6. The compositionsof alloys D-H relate to analysed compositions, while the composition ofalloy I is the nominal composition. Alloys D, E, G and H are commercial,austenitic steels. Alloy F has a composition according to the invention,and alloy I is a commercial, well-known nickel base alloy.

                                      TABLE 6                                     __________________________________________________________________________    Chemical composition, weight-%                                                                           Other                                                                              Ni/Fe--                                       Alloy                                                                             Fe Ni Cr C  Si N Mo Mn elements                                                                           ratio                                         __________________________________________________________________________    D   69.6                                                                              9.6                                                                             18.4                                                                             .06                                                                              1.3                                                                              .15                                                                             .26                                                                               .53                                                                              .04Ce                                                                             .14                                           E   65.5                                                                             10.9                                                                             20.8                                                                             .09                                                                              1.7                                                                              .16                                                                             .24                                                                               .59                                                                              .04Ce                                                                             .17                                           F   36.1                                                                             34.6                                                                             25.8                                                                             .05                                                                              1.5                                                                              .13                                                                             .05                                                                              1.74                                                                              .05Ce                                                                             .96                                           G   53.8                                                                             19.1                                                                             24.7                                                                             .05                                                                              .5 .07                                                                             .25                                                                              1.50                                                                             --   .36                                           H   62.7                                                                             12.6                                                                             22.2                                                                             .06                                                                              .39                                                                              .10                                                                             .37                                                                              1.51                                                                             --   .20                                           I   15.5                                                                             60 23               1.5Al                                                                              3.87                                          __________________________________________________________________________

The materials in all these cases had the shape of plates, and from theseplates coupons were taken, size 10×10×1-2 mm. The coupons were groundand carefully cleaned, whereafter they were subjected to a reducing,carburizing atmosphere at the temperatures 850° C., 950° C., 1050° C.and 1150° C. during a period of exposure which lasted from 20 min to 25h. The reaction gases consisted of 89% H₂ and 11% C₃ H₆, which wasflushed through the furnace at a flow rate of 160 m/min.

The carburization of the studied samples was analysedmetallographically, and the carburization kinetics was found to beparabolic and could be described by the equation x² =2k_(p) t, wherex=the depths of penetration, k_(p) =a rate constant and t=time ofexposure. The obtained data was plotted according to this equation, andthe graphical relations then could be used to estimate the k_(p)-values, which are listed in Table 7 and 8.

It was found through metallurgical studies that the carburization regioncould be devided into two zones. First is the so-called massivecarburization zone which is a zone just beneath the alloy surface. Atgreater depths there is a second zone of caride precipitates along thegrain boundaries. The carburization rate constants, k_(p), are shown inTable 7 for total, i.e. massive plus intergranular carbide formation,and in Table 8 for massive carburization in the surface zone only.

                  TABLE 7                                                         ______________________________________                                        Values of carburization rate constants, k.sub.p (10.sup.3 μm.sup.2         /h)                                                                           for total carburization depths.                                               Temp     Alloy                                                                °C.                                                                             D      E         F    G      H    I                                  ______________________________________                                         850      5.9    1.4      --    3.0   4.0  --                                  950     12.0    2.8       .1   3.8   8.4  .6                                 1050     43.1    48.3     10.8  27.5  38.8 *                                  1150     --     195.7     54.1 196.8  --   *                                  ______________________________________                                         *samples completely carburized                                           

                  TABLE 8                                                         ______________________________________                                        Values of carburization rate constants, k.sub.p (10.sup.3 μm.sup.2         /h)                                                                           for massive carburization.                                                    Temp     Alloy                                                                °C.                                                                             D      E         F    G      H    I                                  ______________________________________                                         850     1.4     .05      --    .8     2.0 --                                  950     4.3    --         .3   4.4    7.0  1.7                               1050     --     14.7       8.4  9.0   15.8  9.4                               1150     --     38.4      11.0 19.5   --   31.2                               ______________________________________                                    

Table 7 and 8 show that alloy F of the invention had the significantlylowest k_(p) -value as far as concerns massive carburization as well astotal carburization.

I claim:
 1. An iron-, nickel-, chromium base alloy having an austeniticstructure, good high temperature features, including a very highresistance oxidization in an oxidizing atmosphere and to carburizationin a carburizing atmosphere at high temperatures, and a high creepfracture resistance, said alloy consisting essentially of the followingcomposition in weight %:

    ______________________________________                                        0.01-0.08       C                                                             1.2-2.0         Si                                                            from traces up to 2                                                                           Mn                                                            22-29           Cr                                                            32-38           Ni                                                            0.01-0.15       rare earth metals                                             0.08-0.25       N                                                             ______________________________________                                    

balance essentially only iron and unavoidable impurities and normallyoccurring accessory elements in normal amounts, said rare earth metalsin combination with the said content of silicon improving the growth ofa protecting SiO₂ -layer on the metal surface, when the metal surface issubjected to high temperatures in an oxidizing atmosphere, whichcounteracts transportation of metal ions out of the alloy, so thatscaling is minimized.
 2. An alloy according to claim 1, wherein thecarbon content of between 0.02 and 0.08%.
 3. An alloy according to claim2, wherein the carbon content is at least 0.035 and not more than0.065%.
 4. An alloy according to claim 1, wherein the silicon content isat least 1.3 and not more than 1.8%.
 5. An alloy according to claim 2,wherein the nitrogen content of between 0.1 and 0.2%.
 6. An alloyaccording to claim 5, wherein the nitrogen content is at least 0.12 andnot more than 0.18%.
 7. An alloy according to claim 1, wherein the rareearth metals content is at least 0.02%.
 8. An alloy according to claim7, wherein the rare earth metals content is at least 0.03%.
 9. An alloyaccording to claim 7, wherein the content of cerium is max 0.1%.
 10. Analloy according to claim 1, wherein the chromium content is between 23and 27%.
 11. An alloy according to claim 1, wherein the nickel contentis between 33 and 37%.
 12. An alloy according to claim 1, wherein themanganese content is between 1.3 and 1.8%.